FIG. 1 is a block diagram showing a configuration of a conventional apparatus for removing shared waves. For example, U.S. Pat. No. 5,363,403 shows a related configuration. In the drawing, reference numeral 1 means an antenna to receive a radio frequency signal, 2 is an amplifier to amplify the radio frequency signal received by the antenna 1, 3 is a local oscillator to generate a local oscillation signal, and 4 is a mixer to mix the radio frequency signal amplified by the amplifier 2 with the local oscillation signal, thereby extracting an intermediate frequency signal.
Reference numeral 5 means a CDMA (Code Division Multiple Access) code generator to generate a shared wave identifying code for identifying a CDMA shared wave, 6 is a correlation detector to detect CDMA#2 signal serving as a shared wave signal through code correlation from the intermediate frequency signal depending upon the shared wave identifying code generated from the CDMA code generator 5, 7 is a demodulator to demodulate information of the CDMA#2 signal serving as the shared wave signal detected by the correlation detector 6, and 8 is a modulator to modulate the internal-oscillated intermediate frequency signal by the shared wave identifying code and the information of the CDMA#2 signal serving as the shared wave signal demodulated by the demodulator 7 depending upon the shared wave identifying code.
Further, reference numeral 9 means a delay circuit to delay the intermediate frequency signal transformed by the mixer 4 for a predetermined time period, 10 is a subtracter to subtract the CDMA#2 signal serving as the shared wave signal modulated by the modulator 8 from the intermediate frequency signal delayed by the delay circuit 9, 11 is a CDMA code generator to generate a desired wave identifying code for identifying a desired wave, 12 is a code correlator to detect CDMA#1 signal serving as a shared wave signal through code correlation from output of the subtracter 10 depending upon the desired wave identifying code generated from the CDMA code generator 11, and 13 is a demodulator to demodulate information of the CDMA#1 signal serving as the shared wave signal detected by the code correlator 12.
FIG. 2 is a block diagram showing a mobile communication system to which the conventional apparatus for removing shared waves is applied. In the drawing, reference numeral 21 means a mobile station equipped with the apparatus for removing shared waves, such as mobile vehicle communication apparatus, or mobile portable communication apparatus. Reference numeral 22 means a radio base station equipped with the apparatus for removing shared waves to communicate with the mobile station 21 through a radio channel 23. Reference numeral 24 means a satellite communication apparatus to communicate with the mobile station 21 through a radio channel 25, and 26 is a satellite communication ground base station equipped with the apparatus for removing shared waves to communicate with the satellite communication apparatus 24 through a satellite line 27.
Further, reference numeral 28 means a public switched telephone network (hereinafter referred to as PSTN), 29 is a wire telephone connected to the PSTN 28, 30 is a private system connected to the PSTN 28, 31 is an exchanger in the private system 30, 32 is a telephone apparatus connected to the exchanger 31, and 33, 34 and 35 are base stations in the private system 30, which are connected to the exchanger 31 and connected to the mobile station 21 through a radio channel 36, and are equipped with the apparatus for removing shared waves.
A description will now be given of the operation.
In radio communication between, for example, the mobile station 21 and the radio base station 22, or the mobile station 21 and the base station 33, that is, in each radio communication concurrently employing a frequency division multiple access method (hereinafter referred to as FDMA method), a multi-carrier time-division multiple access method (hereinafter referred to as TDMA method), a code division multiple access method (hereinafter referred to as CDMA method), and so forth, various types of radio waves are superimposed at the same frequency. Hence, the radio wave received by, for example, the mobile station 21 contains an unnecessary signal (hereinafter referred to as shared wave signal) in addition to a signal to be received (hereinafter referred to as desired wave signal). It is necessary to remove the shared wave signal from the received radio wave in order to extract the desired wave signal with less noise component due to the shared wave signal.
Thus, the apparatus for removing shared waves as shown in FIG. 1 is mounted on, for example, the mobile station 21 to extract the desired wave signal by removing the shared wave signal from the received radio wave.
A description will now be given of the operation of the apparatus for removing shared waves.
First, when the antenna 1 receives the radio frequency signal, the amplifier 2 amplifies the radio frequency signal.
After the amplifier 2 amplifies the radio frequency signal, the mixer 4 extracts the intermediate frequency signal by mixing the local oscillation signal generated from the local oscillator 3 with the radio frequency signal. In this illustration, for the sake of simplicity, it must be noted that the intermediate frequency signal contains CDMA#0 signal and the CDMA#2 signal serving as shared wave signals in addition to the CDMA#1 signal serving as the desired wave signal as shown in FIG. 3. In this connection, FIG. 9 shows a relationship between a frequency and a time slot in the mobile communication system shared between the TDMA method and a CDMA/TDMA method. For example, it is shown that the TDMA#3 and a partial frequency of the CDMA#3 share a time slot.
After the intermediate frequency signal is extracted by the mixer 4 in the above manner, the correlation detector 6 detects the CDMA#2 signal serving as a shared wave signal through code correlation from the intermediate frequency signal depending upon the shared wave identifying code generated from the CDMA code generator 5. That is, as shown in FIG. 4, the CDMA#2 signal serving as the shared wave signal is correlated by regarding the CDMA#0 signal and the CDMA#1 signal contained in the intermediate frequency signal as the noise components.
After the correlation detector 6 detects the CDMA#2 signal serving as the shared wave signal, the CDMA#0 signal and the CDMA#1 signal serving as the noise components are removed. Thus, the demodulator 7 and the modulator 8 demodulate and thereafter modulate the information of the CDMA#2 signal serving as the shared wave signal to output the information to the subtracter 10.
On the other hand, when the intermediate frequency signal is extracted by the mixer 4, the delay circuit 9 delays the intermediate frequency signal for the predetermined time period and thereafter outputs the signal to the subtracter 10 so as to overcome a difference in time between the intermediate frequency signal inputted into the subtracter 10 and the CDMA#2 signal serving as the reproduced shared wave signal.
The subtracter 10 can thereby subtract the CDMA#2 signal serving as the shared wave signal modulated by the modulator 8 from the intermediate frequency signal delayed by the delay circuit 9, thereby removing a shared wave signal component from the intermediate frequency signal (see FIG. 5).
After the subtracter 10 subtracts the CDMA#2 signal serving as the shared wave signal from the intermediate frequency signal, the code correlator 12 detects the CDMA#1 signal serving as the desired wave signal through code correlation from the intermediate frequency signal depending upon the desired wave identifying signal generated from the CDMA code generator 11. That is, as shown in FIG. 6, the CDMA#1 signal serving as the desired wave signal is extracted by regarding the CDMA#0 signal contained in the intermediate frequency signal as the noise component.
After the code correlator 12 detects the CDMA#1 signal serving as the desired wave signal, the demodulator 13 demodulates and outputs the information of the CDMA#1 signal serving as the desired wave signal, resulting in the completion of a series of processing.
The conventional apparatus for removing shared waves has the above structure. Therefore, it is possible to extract the CDMA#1 signal serving as the desired wave signal with relatively high accuracy even if the intermediate frequency signal contains the CDMA#0 signal and the CDMA#2 signal as the shared wave signals in addition to the CDMA#1 signal serving as the desired wave signal. However, as shown in FIG. 7, when the FDMA and the TDMA are contained as shared wave signals, the intermediate frequency signal still contains the FDMA and the TDMA having high signal power even after the subtracter 10 removes the CDMA#2 signal serving as the shared wave signal. Consequently, there is a problem in that, when the code correlator 12 detects the CDMA#1 signal serving as the desired wave signal through the code correlation, the CDMA#1 signal serving as the desired wave signal can not be extracted with high accuracy under the influence of the FDMA or the TDMA.
Further, in the conventional apparatus for removing shared waves, even when transmission path distortion is caused in the radio frequency signal with propagation through a plurality of paths (see FIG. 8), the demodulator 7 demodulates the information of the CDMA#2 signal serving as the shared wave signal without compensating for the transmission path distortion of the CDMA#2 signal serving as the shared wave signal. Hence, even when the modulator 8 modulates and reproduces the information of the CDMA#2 signal serving as the shared wave signal, the transmission path distortion can not accurately be reproduced (there is a difference in waveform between the shared wave signal contained in the intermediate frequency signal outputted from the delay circuit 9 and the shared wave signal outputted from the modulator 8). The subtracter 10 can not accurately remove the CDMA#2 signal serving as the shared wave signal from the intermediate frequency signal. As a result, there is another problem of a lower accuracy with which the CDMA#1 signal serving as the desired wave signal is extracted.
The present invention is made to overcome the above problems. It is an object of the present invention to provide an apparatus for removing shared waves in which a desired wave signal can be extracted with high accuracy even when an intermediate frequency signal contains FDMA or TDMA as a shared wave signal.
It is another object of the present invention to provide an apparatus for removing shared waves in which a desired wave signal can be extracted with high accuracy even when transmission path distortion is caused in a radio frequency signal with propagation through a plurality of paths.